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Re: [Xen-devel] [PATCH] x86/tss: Fix clang build following c/s 7888440625

On 13.08.2019 14:01, Andrew Cooper wrote:
Clang-3.5 from Debian Jessie fails with:

   smpboot.c:829:29: error: statement expression not allowed at file scope
           BUILD_BUG_ON(sizeof(this_cpu(tss_page)) != PAGE_SIZE);
   /local/xen.git/xen/include/asm/percpu.h:14:7: note: expanded from macro
       (*RELOC_HIDE(&per_cpu__##var, get_cpu_info()->per_cpu_offset))
   /local/xen.git/xen/include/xen/compiler.h:98:3: note: expanded from macro
     ({ unsigned long __ptr;                       \
   /local/xen.git/xen/include/xen/lib.h:26:53: note: expanded from macro
   #define BUILD_BUG_ON(cond) ((void)BUILD_BUG_ON_ZERO(cond))
   /local/xen.git/xen/include/xen/lib.h:25:57: note: expanded from macro
   #define BUILD_BUG_ON_ZERO(cond) sizeof(struct { int:-!!(cond); })
   1 error generated.
   /local/xen.git/xen/Rules.mk:202: recipe for target 'smpboot.o' failed

This is obviously a compiler bug because the BUILD_BUG_ON() is not at file
scope.  However, it can be worked around by using a local variable.

It can be worked around, yes, but the result is not identical:

--- a/xen/arch/x86/smpboot.c
+++ b/xen/arch/x86/smpboot.c
@@ -826,9 +826,11 @@ static int setup_cpu_root_pgt(unsigned int cpu)
          rc = clone_mapping(idt_tables[cpu], rpt);
      if ( !rc )
-        BUILD_BUG_ON(sizeof(this_cpu(tss_page)) != PAGE_SIZE);
+        struct tss_page *this_tss = &per_cpu(tss_page, cpu);

Whatever type is compatible with struct tss_page * will now be
permitted on the rhs. Hence ...

-        rc = clone_mapping(&per_cpu(tss_page, cpu).tss, rpt);
+        BUILD_BUG_ON(sizeof(*this_tss) != PAGE_SIZE);

... this sizeof() is not necessarily checking the size of the
per-CPU variable. But anyway - the change has been committed
already, and the chances of this going wrong are low enough, so
please don't treat this as an objection or revert request.


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